The invention relates to an apparatus for detecting information index signal from a magnetically recorded tape, and more particularly, to such apparatus which may be utilized to achieve an automatic retrieval of a desired one of information blocks contained in a magnetically recorded tape.
A magnetic tape such as music tape, data containing tape or the like which contains a plurality of information items may be loaded on a tape recorder to select a desired one of information items in an automatic manner for reproduction thereof. At this end, the tape is usually run at a higher rate by establishing a rapid advance or rewind mode of the tape recorder in order to detect information index signals recorded in the tape until the count of signals detected reaches a preset value, whereupon the tape recorder is automatically set to a playback mode for reproduction of the desired information item.
An area of the tape which is located between adjacent information items and thus is free from any signal may be utilized as information index signal. In a music tape, for example, wherein a plurality of popular musical pieces playing for a period on the order of three minutes are recorded, such signal-free area is provided between adjacent musical pieces and hence can be utilized as information index signal during the rapid advance or rewind operation of the tape. To detect such signal-free area, a detecting head may be brought into abutment against the tape during the rapid advance or rewind mode of the tape. A desired information item can be located by comparing the position in time of a selected signal-free area against a given position in time of a desired information item.
The described apparatus of comparing the time when a selected signal-free area is detected against a given time involves a difficulty in that the intended position in time of a desired information item does not vary linearly with the length of the tape inasmuch as the tape feed rate is not uniform during a rapid advance or rewind mode, but rather the tape take-up shaft which is directly driven causes the tape to run at an increasingly higher rate, resulting in a varying tape running speed with a change in the diameter of the roll of the tape portion disposed on the take-up shaft.
Another difficulty results from the fact that during the time the tape is run at a higher rate, the tape speed undergoes an inconspicuous change, which prevents a positive contact between the tape and the detecting head from being expected. As a consequence, a skip-over occurs, which means a momentary disengagement of the tape from the head surface. In response to a skip-over, the head will produce an output signal which would indicate the occurrence of a signal-free area in the tape, causing a false counting as one of information index signals. A further difficulty occurs with a musical tape in that where a musical piece includes silent portion, it may be detected as a signal-free area between adjacent musical pieces, again causing a false counting.
These disadvantages can be overcome by providing an increased length of signal-free space between adjacent information items and establishing an increased length of time as the threshold for a signal-free area to be detected as such during the rewind or rapid advance of the tape, thus assuring a reliable detection of true signal-free areas. However, when the tape is thus constructed, the tape utilization efficiency is degraded, and the tape being played will be less pleasing and wearisome to a user.